An image recording apparatus of ink-jet recording type prints an image on a recording sheet by ejecting ink drops onto the recording sheet. In such a type of image recording apparatuses, a carriage mounting thereon a print head is supported to be movable in a direction perpendicular to a conveyance direction of a recording sheet. The carriage moves at a specific moving speed with the driving force from a carriage driving motor (hereinafter, referred to as “CR motor”). On the other hand, the recording sheet on which an image is printed is conveyed below the carriage by a conveyance roller. The conveyance roller rotates at a specific angular speed with the driving force from a conveyance roller driving motor (hereinafter, referred to as “LF motor”).
FIG. 9 is a block diagram schematically illustrating a configuration of a feedback control system applied to an image recording apparatus. In FIG. 9, a flow of an electrical signal is indicated by a dashed line and a flow of driving force is indicated by a thick solid line. As shown in FIG. 9, in the control system, an LF motor 211 and a CR motor 221 are controlled by independent control systems, respectively.
As shown in FIG. 9, the driving current ILF is supplied from a motor driver 201 to the LF motor 211 so that a conveyance roller 213 is driven at a specific target angular speed. The driving force of the LF motor 211 is transmitted to the conveyance roller 213 through a gear driving mechanism 212. Accordingly, the conveyance roller 213 rotates. The actual angular speed of the conveyance roller 213 is measured by an optical sensor 215 such as a rotary encoder. The measured angular speed is fed back to the motor driver 201. The motor driver 201 calculates the difference between the fed-back angular speed and the target angular speed and adjusts the value of the driving current ILF so that the actual angular speed gets close to the target angular speed. The CR motor 221 is controlled by a motor driver 202 in feedback manner. Specifically, the driving current ICR is supplied from the motor driver 202 to the CR motor 221 so that the carriage 223 is driven at a specific target moving speed. The driving force of the CR motor 221 is transmitted to the carriage 223 through a belt driving mechanism 222. Accordingly, the carriage 223 is made to move. The actual moving speed of the carriage 223 is measured by an optical sensor 225 such as a linear encoder. The measured moving speed is fed back to the motor driver 202. The motor driver 202 calculates the difference between the fed-back moving speed and the target moving speed and adjusts the driving current ICR so that the actual moving speed gets close to the target moving speed.
In the above-described image recording apparatus, an image printing operation is performed as follows. First, until the leading end of a printing area of a recording sheet reaches the bottom of a print head, the recording sheet is conveyed by the conveyance roller with the driving force of the LF motor. When the leading end of the printing area reaches the bottom of the print head, the LF motor is paused and the recording sheet is stopped. By driving the CR motor in this state, the carriage located at a standby position is made to move. In the course of the movement, the print head selectively ejects ink drops from nozzles while moving along with the carriage. Accordingly, a one-line image is printed on the recording sheet. When the carriage reaches the standby position, the CR motor is paused. Thereafter, the LF motor is intermittently driven and the conveyance roller conveys the recording sheet by a specific width corresponding to one line. Accordingly, the recording sheet is paused every time when the recording sheet is conveyed by the specific width. During the pause, the carriage is made to move and one-line image is printed on the recording sheet. By repeating this operation, a continuous image including specific lines is printed on the recording sheet.
Recently, in such image recording apparatuses, the print head tends to increase in size with the increase of the number of nozzles of the print head or the number of available ink colors. For the purpose of reducing the printing time, the moving speed of the carriage tends to increase. The increase in size of the print head or the increase in moving speed of the carriage causes an increase in load of the CR motor driving the carriage. Accordingly, it is necessary to employ a higher-power motor as the CR motor. When a recording sheet having a thick such as glossy paper having been subjected to a gloss process is conveyed or when the conveyance roller is made to rotate at a high speed to reduce the printing time, the load of the LF motor driving the conveyance roller increases. Accordingly, it is necessary to employ a high-power motor as the LF motor.
As a device for transmitting the driving force of a motor to a specific driven member, JP-A-6-213301 describes a differential gear and JP-A-6-123337 describes a reduction gear. In the former differential gear, two motors are connected through a belt or pulley and the driving force of two motors is output from one output shaft. The latter reduction gear includes plural input shafts to which the rotational force is input, plural output shafts from which the rotational power is output, and a differential gear element interposed therebetween to obtain plural reduced rotation outputs from plural rotation inputs.
When a high-power motor is employed, the motor increases in size, thereby causing an increase in size of the image recording apparatus as a whole. Employing the high-power motor may cause the increase in cost and in power consumption. Accordingly, these days when the decrease in cost, size, and power consumption of the image recording apparatus are strongly required, it is not preferable that high-power motors are employed every driven member since it is opposite to the requirement. The requirement for the decrease in cost, size, and power consumption is also applied to devices having plural motors such as vehicles, industrial robots, portable terminals, and note-type personal computers, as well as the image recording apparatus.